Does the level of Corn Disease justify spraying?

The answer here is no, unless you are working with a susceptible hybrid or variety and the disease is present. The most damaging diseases for Ohio would be Gray Leaf Spot and Northern Corn Leaf Blight. This year does seem to be providing the proper environment for rust and we are seeing high levels reported on popcorn and sweet corn which have low resistance to rust. Most all of the field corn have a good rust resistance built in. Information on corn rust can be found in the factsheet AC 0031-01 Common Corn Rust found below. All other scouting results have shown other common diseases to be at very low levels.

So why spray corn with fungicides? Bob Nielson, Purdue University Corn Agronomist has put together a compilation of some articles on this topic from around the country in an article entitled "Corn Fungicide Hoopla 2008". Dr Nielson provides the definition of hoopla and then links to several articles addressing the third party suggestions and evaluations of the practice.
If you want to get right to the yield data the best summary of midwestern data is assembled by the University of Illinois Making Profitable Fungicide Applications in Corn.

Common Corn Rust

AC-0031-01

Common corn rust rarely causes significant yield losses in Ohio field (dent) corn. However, occasionally field corn has been damaged when environmental conditions favor the development and spread of the disease. Sweet corn is generally more susceptible than field corn. Common rust can usually be found in corn fields sometime during the growing season, but generally does not appear before tasseling. In those years with exceptionally cool summers, and especially on late-planted fields or sweet corn, yield losses may be expected when the leaves above the ears become severely diseased.

Symptoms

Common rust of corn can be easily recognized by the development of dark, reddish-brown pustules scattered over both the upper and lower surface of the corn leaves. Pustules appear oval to elongate in shape, are generally small, less than 1/4 inch long, and are surrounded by the leaf epidermal layer, which appears as a whitish covering. These pustules may appear on any above ground portion of the plant, but are most abundant on the leaves.

Causal Fungus

Puccinia sorghi is the fungus causing common rust in corn. The reddish-brown color of the pustule is actually the coloration of the repeating spore or uredospores. These spores are produced throughout the summer and infect new leaf tissue and are responsible for the spread of the disease. As the corn plant matures the pustules turn a brownish-black color due to the development of the darker pigmented spores or teliospores. In tropical regions teliospores infect the rust's alternate host, wood sorrel (Oxalis species). In temperate areas, including the U.S. cornbelt, the fungus does not infect wood sorrel and the teliospores have no real function.

Disease Cycle

The rust fungus overwinters as uredospores on corn or as other spore forms on wood sorrel in subtropical and tropical regions. Spores capable of infecting corn are blown northward during the growing season and become established on the corn crop. Rust development and spread is favored by prolonged cool temperatures ranging from 60° to 74°F and high relative humidity. When these protracted cool, humid conditions exist, susceptible corn hybrids and sweet corn varieties can become seriously affected.

Control

Although rust is frequently seen on corn in Ohio, very rarely has there been need for control on field corn. In field corn, highly resistant hybrids are available and most hybrids possess some degree of resistance. Pop and sweet corn can be quite susceptible. In seasons where considerable rust is present on the lower leaves prior to silking and the weather forecast is for unseasonably cool, wet weather, chemical control may be necessary. Fungicides are available for rust control. Consult label recommendations for rates and application timing. Early application is necessary for effective disease control.

Crop Progress Pest Report as of 7/11/08-Fulton County

Thursday and Friday afternoon last week I traveled the route of Road B and Road L between to Lucas and Williams County lines. I visited the same 35 Corn, 42 Soybean and 25 wheat fields that have been on the route. I was unable to get to a couple fields due to an accident investigation nearby.

Corn has progressed to the early pollination stages. In fact 12 (34%) of the fields were in the R1 stage (silking) , 9 (25%) fields were in VT (tassel) and 14 (40%) fields were in late vegetative (V10-V15). The crop generally looks good but there are water damage areas. Insect pressure is low with some European Corn Borer that had entered the stalk (4 of 35) and very limited numbers of Corn Rootworm beetles (2 of 35). It maybe just a little bit early to evaluate rootworm numbers but we after historically high numbers in 2007 it was noteworthy that very few were found. From a disease standpoint the only disease noted was leaf rust and it was generally just a pustule or two on a leaf and extremely limited in intensity.

Soybeans have generally taken this wet weather harder than the corn. Much of the worst loss of crop was east of 13. One 80 acre field was around 50% lost to flooding from July 2 and 3 rains. From a growth stage standpoint there were 5 (12%) fields in the R1 (beginning bloom), 34 (81%) in R2 (full bloom) and 3 (7%) in R3 (beginning pod). Digging up plants you were starting to see significant regrowth of the roots and some new nodulation. A few leaf feeding insects were present Japanese beetle and been leaf beetle could be found. From a disease standpoint brown spot was the most common disease with some phytopthora in a couple fields. More information on scouting for soybean disease can be found at http://ohioline.osu.edu/b827/0003.html

Wheat was harvested in all 28 wheat fields I had been following. There were still a couple wheat fields not harvested in between stops but we were approaching completion. Wheat yield reports appear to be in the 70-95 range with more it seems in the mid 70-80 range. It seems that 75% of the field were or will have the straw removed. From a nutrient standpoint a ton of straw contains 11 pounds of N, 3 pound of P2O5 and 15 pounds of K2O which at today's fertilizer prices are about $20 a ton plus the value of the organic matter. A general straw yield is 2.7 tons per acre.

Wheat Harvest Between Rainfall

Wheat harvest began in Defiance County over the Independence Day weekend. A few well-drained fields were dry enought to support harvest equipment whereas, wetter fields wait for improved field conditions and for the crop to rippen. Early report provide good yields in the range of 70-85 bushel per acre with excellent test weight. Grain moisture is reported in the range of 14-16% with a few growers harvesting wheat a bit sooner than normal to plant double crop soybeans.

Forage harvest continues to be a challenge with the recent rainfall. Alfalfa producers should scout for potato leaf hoppers during the regrowth period following 1st cutting.

Soybean plants that remain in saturated soils are beginning to show phytophthora stem rot. Classic sympotoms are plants, scattered throughout the wet areas with the chocolate brown canker moving up the plant. The upper part of the plant will turn yellow and wilt.

Corn has entered a period of rapid growth and will soon be determining the number of kernals per row on the ear. This is usually complete about one week before silking (R1) or about the V17 stage.

July 2-3 Rains bring flooding concerns

Today's rainfall report are astonishing. It appears the heaviest rainfall was north of the turnpike. Rainfall in the Wauseon area was 2.5 inches but north of the turnpike reports of 5-7.5 inches are common across much of northern Fulton County. Below are articles reprint from 2003 edition of CORN newsletter on flooding in corn and 2006 CORN related to soybean and flooding injury.

Early Season Ponding and Flooding Injury to Corn-Dr Peter Thomison

Recent heavy rains have generated questions about flooding and ponding injury in corn. The following are some tips to consider when evaluating possible damage from water saturated soil conditions.

The extent to which flooding injures corn is determined by several factors including: (1) plant stage of development when flooding occurs, (2) duration of flooding and (3) air/soil temperatures. Prior to the 6-leaf collar stage (as measured by visible leaf collars) or when the growing point is at or below the soil surface, corn can usually survive only 2 to 4 days of flooded conditions. The oxygen supply in the soil is depleted after about 48 hours in a flooded soil. Without oxygen, the plant cannot perform critical life sustaining functions; e.g. nutrient and water uptake is impaired, root growth is inhibited, etc. If temperatures are warm during flooding (greater than 77 degrees F) plants may not survive 24-hours. Cooler temperatures prolong survival. Once the growing point is above the water level the likelihood for survival improves greatly.

Even if flooding doesn't kill plants outright, it may have a long term negative impact on crop performance. Excess moisture during the early vegetative stages retards corn root development. As a result, plants may be subject to greater injury during a dry summer because root systems are not sufficiently developed to access available subsoil water. Flooding and ponding can also result in losses of nitrogen through denitrification and leaching.

If flooding in corn lasts less than 48 hours, crop injury should be limited. To confirm plant survival, check the color of the growing point. It should be white to cream colored, while a darkening and/or softening usually precedes plant death. Also look for new leaf growth 3 to 5 days after water drains from the field. Sometimes the growing point is killed by bacterial infections during and after flooding, but plant growth continues in the form of non-productive tillers (suckers).

Disease problems that become greater risks due to flooding and cool temperatures include pythium, corn smut, and crazy top. Despite fungicide seed treatments, pythium root rot contributed to serious stand reductions in many corn fields last year. The fungus that causes crazy top depends on saturated soil conditions to infect corn seedlings. There is limited hybrid resistance to these diseases and predicting damage from corn smut and crazy top is difficult until later in the growing season.

Flooding Injury to Soybeans - Anne Dorrance

Several past studies have evaluated the effects flooding on soybeans. Research projects led by Dr. Tara Van Toai of the USDA-ARS and Department of Horticulture and Crop Science. In one study, soybean plants at the V2 and V3 stages were flooded naturally due to excessive rainfall, something we are all familiar with. Two 9-m wide transects across the flooded area within each field were divided into plots of 9 m by 9 m according to flooding duration: no flooding, 1 to 3 d, 4 to 6 d, and 6 to 8 d. Yield was reduced primarily due to reduced plant populations, shorter plants and fewer pods developing per plant. Other studies showed yield losses of 20% when soybean fields were flooded for greater than 2 days, but it should be noted they still had a yield. Some varieties can tolerate flooding more than others. For those fields with greater than two days of total saturation, ponding, can expect to see some long term effects. Secondary factors, such as root rots, will also contribute to the damage. Growers should note the varieties and compared varieties that were flooded for the same length of time. Some varieties will be impacted to a greater degree than others.

Head Scab Present but Low Counts

On June 24 & 25, twelve fields across Fulton County were scouted for head scab. Head Scab causes part of or all of an individual head to be bleached out on close observation you may see a orangish-pink growth where the kernel is attached and the lower glume. Every field had some scabby heads in them. The average was 5.6% of the head affected. The range was 1-15%. Counts were made per foot of row. The average head count per foot of row was 38 heads. The head count compares with previous years 2006-38.8 and 2005-39.



Two fields had noticeable levels of Glume blotch which is a black coloration of the glumes that surround the kernel. This particular disease can impair grain fill and result in lower test weights.